Thermodynamic modeling of phase equilibria of clathrate hydrates formed from CH4, CO2, C2H6, N2 and C3H8, with different equations of state

A thermodynamic model to predict three phase (L-H-V and I-H-V) equilibria of gas hydrates is presented. In this model we have employed a fugacity based approach where the hydrate phase is modeled using van der Waals-Platteeuw solid solution theory and the liquid phase activity coefficients are determined from the modified UNIFAC method. For the vapour phase fugacity calculations we have investigated three equations of state (EOS): Peng-Robinson-Stryjek-Vera (PRSV), Patel-Teja (PT) and Soave-Redlich-Kwong (SRK). This model employs only parameters reported in the literature. The coexistence pressures predicted by our model for the sI hydrates of methane, carbon dioxide and ethane are in reasonable agreement with experiments, whereas our model overestimates the coexistence pressures for the sII clathrates of nitrogen and propane. The predicted cage occupancies are found to increase with increasing temperature in the L-H-V equilibria. For I-H-V equilibria the cage occupancy is observed to decrease with temperature. We have also estimated the solubility of each guest in the liquid phase (for L-H-V equilibria) using the Henry's law. The solubilities predicted using all three EOS are in good agreement for all guest molecules, with the exception of nitrogen where at relatively higher temperatures the estimates from the PRSV EOS are noticeably lower than the corresponding predictions from the PT and SRK EOS.